ECMAScript 5.1, the latest edition of the standard upon which JavaScript is based, was approved in June 2011.

The JavaScript runtime used in the latest versions of Mozilla projects including both Firefox and Thunderbird has full support for ECMAScript 5.1 features. This article covers the features supported by different versions of Mozilla's JavaScript runtime.

Supported features

Added in JavaScript 1.8.5 (Gecko 2, Firefox 4 and later)

Firefox 4 has full ECMAScript 5 support including the Object.* methods and strict mode. See New in JavaScript 1.8.5.